Added fontSizeTransition param

[Seferin]

Related to issue #299

[Seferin]

A quick demo of functionality with the Iris datset.

str_list <- list(name = "root ('data.frame':\t150 obs. of 5 variables:)",
children = list(list(name = "Sepal.Length (num)"), list(name = "Sepal.Width (num)"),
list(name = "Petal.Length (num)"), list(name = "Petal.Width (num)"),
list(name = "Species (Factor w/ 3 levels "setosa","versicolor",)")))

diagonalNetwork(str_list, fontSize = 35, fontSizeTransition = 45)